The simple adder constructed in Code is composed of a "half adder" which adds two bits into a sum and carry out:
Two of these are then wired together to make a "full adder" which also takes into account an incoming carry:
Then, multiple of these full adders are wired together to create a circuit which can add larger binary numbers:
This circuit adds two eight bit numbers together, however it does not do so very efficiently. Remember that logic gates do not work instantaneously. If we change the input to a gate, there is some delay before the output changes.
For this adder to give us a full result, the carry has to propagate from the right-most full adder all the way to the left-most one. This type of adder is called a "ripple carry" adder for that reason.
A Logisim version of this adder is available at adder.circ.
Copyright © 2025 Ian Finlayson | Licensed under a Creative Commons BY-NC-SA 4.0 License.